<p>A prototype development environment for the Mars maintainable real-time system is described. The Mars design environment supports the implementation of critical real-time tasks with time editing. It uses an experimental real-time language, Modula-R, which is based on Modula-2. The Mars real-time architecture, which is the target architecture of the Mars environment, is described. To illustrate the environment, the development of a simple real-time algorithm that implements insertion sort for an array is presented.</p>